Toward Orthogonal Self-Assembly of Redox Active Molecules on Pt and Au: Selective Reaction of Disulfide with Au and Isocyanide with Pt

Surfaces of platinum and gold adsorb 12-ferrocenyldodecyl isocyanide, I, and bis[10-(ferrocenylcarbonyl)-decyl] disulfide, II, and yield self-assembled, redox active monolayers incorporating ferrocene groups. The values of E_1/2 for alkylferrocenyl centers from I are ~300 mV more negative than acylferrocenyl centers from II. This difference allows easy electrochemical measurement of surface concentrations of ferrocene centers from I and II on Pt or Au. I adsorbs selectively on Pt or Au compared to II, but with different degrees of selectivity. Further, for Pt the selectivity depends on the surface pretreatment. The affinity of II for a Pt surface first exposed to an O₂ plasma (oxidized Pt) is lower than that for a Pt surface exposed first to an O₂ plasma and then to a H₂ plasma (reduced Pt), but the two plasma pretreatments of Pt do not affect the binding of I on this metal. The same two plasma pretreatments of Au do not influence the selectivity for binding I vs II; the selectivity for binding I vs II on Au is ~8:1. By use of the O₂ plasma pretreatment and a 40:1 solution of II:I, self-assembly of II onto Au and I onto Pt can be effected when the two samples are derivatized in the same solution.